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B2 has an interpreted, procedural language. Statements in
b2 are rule (procedure) definitions, rule invocations, flow-of-control
structures, variable assignments, and sundry language support.
B2 treats its input files as whitespace-separated tokens,
with two exceptions: double quotes (") can enclose whitespace to embed
it into a token, and everything between the matching curly braces ({}) in
the definition of a rule action is treated as a single string. A backslash
(\) can escape a double quote, or any single whitespace character.
B2 requires whitespace (blanks, tabs, or newlines) to
surround all tokens, including the colon (:) and semicolon (;) tokens.
B2 keywords (an mentioned in this document) are reserved
and generally must be quoted with double quotes (") to be used as arbitrary
tokens, such as variable or target names.
Comments start with the # character and extend until the
end of line.
The essential b2 data entity is a target. Build targets
are files to be updated. Source targets are the files used in updating built
targets. Built targets and source targets are collectively referred to as
file targets, and frequently built targets are source targets for other built
targets. Pseudotargets are symbols representing dependencies on other targets,
but which are not themselves associated with any real file.
A file target's identifier is generally the file's name, which can be absolutely
rooted, relative to the directory of b2's invocation,
or simply local (no directory). Most often it is the last case, and the actual
file path is bound using the $(SEARCH) and $(LOCATE)
special variables. See SEARCH
and LOCATE Variables below. A local filename is optionally qualified
with grist, a string value used to assure uniqueness. A file target with
an identifier of the form file(member) is a library
member (usually an ar(1) archive on Unix).
Whenever a target is bound to a location in the filesystem, Boost Jam will
look for a variable called BINDRULE (first "on"
the target being bound, then in the global module). If non-empty, =$(BINDRULE[1])=
names a rule which is called with the name of the target and the path it
is being bound to. The signature of the rule named by =$(BINDRULE[1])=
should match the following:
rule bind-rule ( target : path )
This facility is useful for correct header file scanning, since many compilers
will search for #include
files first in the directory containing the file doing the #include directive. $(BINDRULE)
can be used to make a record of that directory.
The basic b2 language entity is called a rule. A rule
is defined in two parts: the procedure and the actions. The procedure is
a body of jam statements to be run when the rule is invoked; the actions
are the OS shell commands to execute when updating the built targets of the
rule.
Rules can return values, which can be expanded into a list with "[ rule args ... ]". A rule's value is the value of its last statement, though only the following statements have values: 'if' (value of the leg chosen), 'switch' (value of the case chosen), set (value of the resulting variable), and 'return' (value of its arguments). Note that 'return' doesn't actually cause a return, i.e., is a no-op unless it is the last statement of the last block executed within rule body.
The b2 statements for defining and invoking rules are
as follows:
Define a rule's procedure, replacing any previous definition.
rule rulename { statements }
Define a rule's updating actions, replacing any previous definition.
actions [ modifiers ] rulename { commands }
Invoke a rule.
rulename field1 : field2 : ... : fieldN ;
Invoke a rule under the influence of target's specific variables..
on target rulename field1 : field2 : ... : fieldN ;
Used as an argument, expands to the return value of the rule invoked.
[ rulename field1 : field2 : ... : fieldN ] [ on target rulename field1 : field2 : ... : fieldN ]
A rule is invoked with values in field1 through fieldN.
They may be referenced in the procedure's statements as $(1)
through $(N) (9 max), and the first
two only may be referenced in the action's commands
as $(1) and $(2). $(<)
and $(>) are synonymous with $(1)
and $(2).
Rules fall into two categories: updating rules (with actions), and pure procedure
rules (without actions). Updating rules treat arguments $(1)
and $(2) as built targets and sources, respectively, while
pure procedure rules can take arbitrary arguments.
When an updating rule is invoked, its updating actions are added to those
associated with its built targets ($(1)) before the rule's
procedure is run. Later, to build the targets in the updating phase, commands
are passed to the OS command shell, with $(1) and $(2)
replaced by bound versions of the target names. See Binding above.
Rule invocation may be indirected through a variable:
$(var) field1 : field2 : ... : fieldN ; on target $(var) field1 : field2 : ... : fieldN ; [ $(var) field1 : field2 : ... : fieldN ] [ on target $(var) field1 : field2 : ... : fieldN ]
The variable's value names the rule (or rules) to be invoked. A rule is invoked
for each element in the list of $(var)'s
values. The fields field1 : field2
: ... are passed as arguments for each invokation.
For the [ ... ] forms, the return value is the concatenation of the return
values for all of the invocations.
The following action modifiers are understood:
actions bind vars
$(vars) will be replaced
with bound values.
actions existing
$(>) includes only source targets currently
existing.
actions ignoreThe return status of the commands is ignored.
actions piecemeal
commands are repeatedly invoked with a subset of $(>)
small enough to fit in the command buffer on this OS.
actions quietlyThe action is not echoed to the standard output.
actions together
The $(>) from multiple invocations of the same
action on the same built target are glommed together.
actions updated
$(>) includes only source targets themselves
marked for updating.
You can describe the arguments accepted by a rule, and refer to them by name within the rule. For example, the following prints "I'm sorry, Dave" to the console:
rule report ( pronoun index ? : state : names + )
{
local he.suffix she.suffix it.suffix = s ;
local I.suffix = m ;
local they.suffix you.suffix = re ;
ECHO $(pronoun)'$($(pronoun).suffix) $(state), $(names[$(index)]) ;
}
report I 2 : sorry : Joe Dave Pete ;
Each name in a list of formal arguments (separated by ":"
in the rule declaration) is bound to a single element of the corresponding
actual argument unless followed by one of these modifiers:
|
Symbol |
Semantics of preceding symbol |
|---|---|
|
|
optional |
|
|
Bind to zero or more unbound elements of the actual argument.
When |
|
|
Bind to one or more unbound elements of the actual argument. |
The actual and formal arguments are checked for inconsistencies, which
cause b2 to exit with an error code:
### argument error # rule report ( pronoun index ? : state : names + ) # called with: ( I 2 foo : sorry : Joe Dave Pete ) # extra argument foo ### argument error # rule report ( pronoun index ? : state : names + ) # called with: ( I 2 : sorry ) # missing argument names
If you omit the list of formal arguments, all checking is bypassed as in "classic" Jam. Argument lists drastically improve the reliability and readability of your rules, however, and are strongly recommended for any new Jam code you write.
B2 has a growing set of built-in rules, all of which
are pure procedure rules without updating actions. They are in three groups:
the first builds the dependency graph; the second modifies it; and the
third are just utility rules.
rule DEPENDS ( targets1 * : targets2 * )
Builds a direct dependency: makes each of targets1 depend on each of targets2. Generally, targets1 will be rebuilt if targets2 are themselves rebuilt or are newer than targets1.
rule INCLUDES ( targets1 * : targets2 * )
Builds a sibling dependency: makes any target that depends on any of targets1 also depend on each of targets2. This reflects the dependencies that arise when one source file includes another: the object built from the source file depends both on the original and included source file, but the two sources files don't depend on each other. For example:
DEPENDS foo.o : foo.c ; INCLUDES foo.c : foo.h ;
"foo.o" depends on "foo.c"
and "foo.h" in this example.
The six rules ALWAYS, LEAVES,
NOCARE, NOTFILE, NOUPDATE,
and TEMPORARY modify the dependency graph so that
b2 treats the targets differently during its target
binding phase. See Binding above. Normally, b2 updates
a target if it is missing, if its filesystem modification time is older
than any of its dependencies (recursively), or if any of its dependencies
are being updated. This basic behavior can be changed by invoking the
following rules:
rule ALWAYS ( targets * )
Causes targets to be rebuilt regardless of whether
they are up-to-date (they must still be in the dependency graph). This
is used for the clean and uninstall targets, as they have no dependencies
and would otherwise appear never to need building. It is best applied
to targets that are also NOTFILE targets, but it
can also be used to force a real file to be updated as well.
rule LEAVES ( targets * )
Makes each of targets depend only on its leaf sources, and not on any intermediate targets. This makes it immune to its dependencies being updated, as the "leaf" dependencies are those without their own dependencies and without updating actions. This allows a target to be updated only if original source files change.
rule NOCARE ( targets * )
Causes b2 to ignore targets
that neither can be found nor have updating actions to build them.
Normally for such targets b2 issues a warning and
then skips other targets that depend on these missing targets. The
HdrRule in Jambase uses NOCARE
on the header file names found during header file scanning, to let
b2 know that the included files may not exist. For
example, if an #include
is within an #ifdef,
the included file may not actually be around.
For targets with build actions: if their build actions exit with a nonzero return code, dependent targets will still be built.
rule NOTFILE ( targets * )
Marks targets as pseudotargets and not real files.
No timestamp is checked, and so the actions on such a target are only
executed if the target's dependencies are updated, or if the target
is also marked with ALWAYS. The default b2
target "all" is a pseudotarget. In Jambase,
NOTFILE is used to define several addition convenient
pseudotargets.
rule NOUPDATE ( targets * )
Causes the timestamps on targets to be ignored.
This has two effects: first, once the target has been created it will
never be updated; second, manually updating target will not cause other
targets to be updated. In Jambase, for example,
this rule is applied to directories by the MkDir
rule, because MkDir only cares that the target directory
exists, not when it has last been updated.
rule TEMPORARY ( targets * )
Marks targets as temporary, allowing them to be
removed after other targets that depend upon them have been updated.
If a TEMPORARY target is missing, b2
uses the timestamp of the target's parent. Jambase
uses TEMPORARY to mark object files that are archived
in a library after they are built, so that they can be deleted after
they are archived.
rule FAIL_EXPECTED ( targets * )
For handling targets whose build actions are expected to fail (e.g.
when testing that assertions or compile-time type checking work properly),
Boost Jam supplies the FAIL_EXPECTED rule in the
same style as NOCARE, et. al. During target updating,
the return code of the build actions for arguments to FAIL_EXPECTED
is inverted: if it fails, building of dependent targets continues as
though it succeeded. If it succeeds, dependent targets are skipped.
rule RMOLD ( targets * )
B2 removes any target files that may exist on disk
when the rule used to build those targets fails. However, targets whose
dependencies fail to build are not removed by default. The RMOLD
rule causes its arguments to be removed if any of their dependencies
fail to build.
rule ISFILE ( targets * )
ISFILE marks targets as required to be files. This
changes the way b2 searches for the target such
that it ignores matches for file system items that are not files, like
directories. This makes it possible to avoid #include
"exception" matching
if one happens to have a directory named exception in the header search
path.
This is currently not fully implemented.
The two rules ECHO and EXIT are
utility rules, used only in b2's parsing phase.
rule EXIT ( message * : result-value ? )
Blurts out the message to stdout and then exits with a failure status if no result-value is given, otherwise it exits with the given result-value.
"Echo", "echo",
"Exit", and "exit"
are accepted as aliases for ECHO and EXIT,
since it is hard to tell that these are built-in rules and not part
of the language, like "include".
The GLOB rule does filename globbing.
rule GLOB ( directories * : patterns * : downcase-opt ? )
Using the same wildcards as for the patterns in the switch statement.
It is invoked by being used as an argument to a rule invocation inside
of "[ ]". For example: "FILES
= [ GLOB dir1 dir2 : *.c *.h ]" sets FILES
to the list of C source and header files in dir1
and dir2. The resulting filenames are the full pathnames,
including the directory, but the pattern is applied only to the file
name without the directory.
If downcase-opt is supplied, filenames are converted to all-lowercase before matching against the pattern; you can use this to do case-insensitive matching using lowercase patterns. The paths returned will still have mixed case if the OS supplies them. On Windows NT and Cygwin, filenames are always downcased before matching.
The MATCH rule does pattern matching.
rule MATCH ( regexps + : list * )
Matches the egrep(1) style regular expressions
regexps against the strings in list.
The result is a list of matching () subexpressions
for each string in list, and for each regular
expression in regexps.
rule BACKTRACE ( )
Returns a list of quadruples: filename line module rulename..., describing each shallower level of the call stack. This rule can be used to generate useful diagnostic messages from Jam rules.
rule UPDATE ( targets * )
Classic jam treats any non-option element of command line as a name
of target to be updated. This prevented more sophisticated handling
of command line. This is now enabled again but with additional changes
to the UPDATE rule to allow for the flexibility
of changing the list of targets to update. The UPDATE rule has two
effects:
If no target was specified with the UPDATE rule,
no targets will be updated. To support changing of the update list
in more useful ways, the rule also returns the targets previously in
the update list. This makes it possible to add targets as such:
local previous-updates = [ UPDATE ] ; UPDATE $(previous-updates) a-new-target ;
rule W32_GETREG ( path : data ? )
Defined only for win32 platform. It reads the registry of Windows. 'path' is the location of the information, and 'data' is the name of the value which we want to get. If 'data' is omitted, the default value of 'path' will be returned. The 'path' value must conform to MS key path format and must be prefixed with one of the predefined root keys. As usual,
HKLM' is equivalent to 'HKEY_LOCAL_MACHINE'.
HKCU' is equivalent to 'HKEY_CURRENT_USER'.
HKCR' is equivalent to 'HKEY_CLASSES_ROOT'.
Other predefined root keys are not supported.
Currently supported data types : 'REG_DWORD', 'REG_SZ',
'REG_EXPAND_SZ', 'REG_MULTI_SZ'.
The data with 'REG_DWORD' type will be turned into
a string, 'REG_MULTI_SZ' into a list of strings,
and for those with 'REG_EXPAND_SZ' type environment
variables in it will be replaced with their defined values. The data
with 'REG_SZ' type and other unsupported types will
be put into a string without modification. If it can't receive the
value of the data, it just return an empty list. For example,
local PSDK-location = [ W32_GETREG HKEY_LOCAL_MACHINE\\SOFTWARE\\Microsoft\\MicrosoftSDK\\Directories : "Install Dir" ] ;
rule W32_GETREGNAMES ( path : result-type )
Defined only for win32 platform. It reads the registry of Windows.
'path' is the location of the information, and
'result-type' is either 'subkeys'
or 'values'. For more information on 'path'
format and constraints, please see W32_GETREG.
Depending on 'result-type', the rule returns one of the following:
subkeysNames of all direct subkeys of 'path'.
valuesNames of values contained in registry key given by 'path'. The "default" value of the key appears in the returned list only if its value has been set in the registry.
If 'result-type' is not recognized, or requested data cannot be retrieved, the rule returns an empty list. Example:
local key = "HKEY_LOCAL_MACHINE\\SOFTWARE\\Microsoft\\Windows\\CurrentVersion\\App Paths" ;
local subkeys = [ W32_GETREGNAMES "$(key)" : subkeys ] ;
for local subkey in $(subkeys)
{
local values = [ W32_GETREGNAMES "$(key)\\$(subkey)" : values ] ;
for local value in $(values)
{
local data = [ W32_GETREG "$(key)\\$(subkey)" : "$(value)" ] ;
ECHO "Registry path: " $(key)\\$(subkey) ":" $(value) "=" $(data) ;
}
}
rule SHELL ( command : * )
SHELL executes command, and
then returns the standard output of command.
SHELL only works on platforms with a popen()
function in the C library. On platforms without a working popen()
function, SHELL is implemented as a no-op. SHELL
works on Unix, MacOS X, and most Windows compilers. SHELL
is a no-op on Metrowerks compilers under Windows. There is a variable
set of allowed options as additional arguments:
exit-statusIn addition to the output the result status of the executed command is returned as a second element of the result.
no-outputDon't capture the output of the command. Instead an empty ("") string value is returned in place of the output.
Because the Perforce/Jambase defines a SHELL rule
which hides the builtin rule, COMMAND can be used
as an alias for SHELL in such a case.
rule SPLIT_BY_CHARACTERS ( string : delimiters )
SPLIT_BY_CHARACTERS splits the specified string
on any delimiter character present in delimiters
and returns the resulting list.
rule PRECIOUS ( targets * )
The PRECIOUS rule specifies that each of the targets
passed as the arguments should not be removed even if the command updating
that target fails.
rule PAD ( string : width )
If string is shorter than width characters, pads it with whitespace characters on the right, and returns the result. Otherwise, returns string unmodified.
rule FILE_OPEN ( filename : mode )
The FILE_OPEN rule opens the specified file and
returns a file descriptor. The mode parameter
can be either "w" or "r". Note that at present,
only the UPDATE_NOW rule can use the resulting file
descriptor number.
rule UPDATE_NOW ( targets * : log ? : ignore-minus-n ? )
The UPDATE_NOW caused the specified targets to be
updated immediately. If update was successfull, non-empty string is
returned. The log parameter, if present, specifies
a descriptor of a file where all output from building is redirected.
If the ignore-minus-n parameter is specified,
the targets are updated even if the -n parameter
is specified on the command line.
B2 has several simple flow-of-control statements:
for var in list { statements }
Executes statements for each element in list, setting the variable var to the element value.
if cond { statements } [ else { statements } ]
Does the obvious; the else clause is optional. cond
is built of:
atrue if any a element is a non-zero-length string
a = blist a matches list b string-for-string
a != blist a does not match list b
a < ba[i] string is less than b[i] string, where i is first mismatched element in lists a and b
a <= bevery a string is less than or equal to its b counterpart
a > ba[i] string is greater than b[i] string, where i is first mismatched element
a >= bevery a string is greater than or equal to its b counterpart
a in btrue if all elements of a can be found in b, or if a has no elements
! condcondition not true
cond && condconjunction
cond || conddisjunction
( cond )precedence grouping
include file ;
Causes b2 to read the named file.
The file is bound like a regular target (see Binding
above) but unlike a regular target the include file
cannot be built.
The include file is inserted into the input stream during
the parsing phase. The primary input file and all the included file(s) are
treated as a single file; that is, b2 infers no scope
boundaries from included files.
local vars [ = values ] ;
Creates new vars inside to the enclosing {}
block, obscuring any previous values they might have. The previous values
for vars are restored when the current block ends. Any rule called or file
included will see the local and not the previous value (this is sometimes
called Dynamic Scoping). The local statement may appear anywhere, even outside
of a block (in which case the previous value is restored when the input ends).
The vars are initialized to values
if present, or left uninitialized otherwise.
return values ;
Within a rule body, the return statement sets the return value for an invocation of the rule. It does not cause the rule to return; a rule's value is actually the value of the last statement executed, so a return should be the last statement executed before the rule "naturally" returns.
switch value { case pattern1 : statements ; case pattern2 : statements ; ... }
The switch statement executes zero or one of the enclosed statements, depending on which, if any, is the first case whose pattern matches value. The pattern values are not variable-expanded. The pattern values may include the following wildcards:
?match any single character
*match zero or more characters
[chars]match any single character in chars
[^chars]match any single character not in chars
\xmatch x (escapes the other wildcards)
while cond { statements }
Repeatedly execute statements while cond remains true upon entry. (See the description of cond expression syntax under if, above).
B2 variables are lists of zero or more elements, with
each element being a string value. An undefined variable is indistinguishable
from a variable with an empty list, however, a defined variable may have
one more elements which are null strings. All variables are referenced as
$(variable).
Variables are either global or target-specific. In the latter case, the variable takes on the given value only during the updating of the specific target.
A variable is defined with:
variable = elements ; variable += elements ; variable on targets = elements ; variable on targets += elements ; variable default = elements ; variable ?= elements ;
The first two forms set variable globally. The third
and forth forms set a target-specific variable. The =
operator replaces any previous elements of variable
with elements; the += operation adds
elements to variable's list of
elements. The final two forms are synonymous: they set variable
globally, but only if it was previously unset.
Variables referenced in updating commands will be replaced with their values;
target-specific values take precedence over global values. Variables passed
as arguments ($(1) and $(2)) to actions
are replaced with their bound values; the "bind"
modifier can be used on actions to cause other variables to be replaced with
bound values. See Action Modifiers above.
B2 variables are not re-exported to the environment of
the shell that executes the updating actions, but the updating actions can
reference b2 variables with $(variable).
During parsing, b2 performs variable expansion on each
token that is not a keyword or rule name. Such tokens with embedded variable
references are replaced with zero or more tokens. Variable references are
of the form $(v) or $(vm),
where v is the variable name, and m
are optional modifiers.
Variable expansion in a rule's actions is similar to variable expansion in statements, except that the action string is tokenized at whitespace regardless of quoting.
The result of a token after variable expansion is the product of the components of the token, where each component is a literal substring or a list substituting a variable reference. For example:
$(X) -> a b c t$(X) -> ta tb tc $(X)z -> az bz cz $(X)-$(X) -> a-a a-b a-c b-a b-b b-c c-a c-b c-c
The variable name and modifiers can themselves contain a variable reference, and this partakes of the product as well:
$(X) -> a b c $(Y) -> 1 2 $(Z) -> X Y $($(Z)) -> a b c 1 2
Because of this product expansion, if any variable reference in a token is undefined, the result of the expansion is an empty list. If any variable element is a null string, the result propagates the non-null elements:
$(X) -> a "" $(Y) -> "" 1 $(Z) -> -$(X)$(Y)- -> -a- -a1- -- -1- -$(X)$(Z)- ->
A variable element's string value can be parsed into grist and filename-related components. Modifiers to a variable are used to select elements, select components, and replace components. The modifiers are:
[n]Select element number n (starting at 1). If the variable contains fewer than n elements, the result is a zero-element list. n can be negative in which case the element number n from the last leftward is returned.
[n-m]Select elements number n through m. n and m can be negative in which case they refer to elements counting from the last leftward.
[n-]Select elements number n through the last. n can be negative in which case it refers to the element counting from the last leftward.
:BSelect filename base.
:SSelect (last) filename suffix.
:MSelect archive member name.
:DSelect directory path.
:PSelect parent directory.
:GSelect grist.
:UReplace lowercase characters with uppercase.
:LReplace uppercase characters with lowercase.
:TConverts all back-slashes ("\") to forward slashes ("/"). For example
x = "C:\\Program Files\\Borland" ; ECHO $(x:T) ;
prints "C:/Program Files/Borland"
:W
When invoking Windows-based tools from Cygwin
it can be important to pass them true windows-style paths. The :W
modifier, under Cygwin only, turns
a cygwin path into a Win32 path using the cygwin_conv_to_win32_path
function. On other platforms, the string is unchanged. For example
x = "/cygdrive/c/Program Files/Borland" ; ECHO $(x:W) ;
prints "C:\Program Files\Borland" on
Cygwin
:charsSelect the components listed in chars.
:G=gristReplace grist with grist.
:D=pathReplace directory with path.
:B=baseReplace the base part of file name with base.
:S=sufReplace the suffix of file name with suf.
:M=memReplace the archive member name with mem.
:R=rootPrepend root to the whole file name, if not already rooted.
:E=valueAssign value to the variable if it is unset.
:J=joinvalConcatentate list elements into single element, separated by joinval'.
On VMS, $(var:P) is the parent directory of $(var:D).
Boost Jam allows you to declare a local for loop control variable right in the loop:
x = 1 2 3 ;
y = 4 5 6 ;
for local y in $(x)
{
ECHO $(y) ; # prints "1", "2", or "3"
}
ECHO $(y) ; # prints "4 5 6"
During expansion of expressions b2 also looks for subexpressions
of the form =@(filename:Efilecontents) and replaces
the expression with filename after creating the given
file with the contents set to filecontents. This is
useful for creating compiler response files, and other "internal"
files. The expansion works both during parsing and action execution. Hence
it is possible to create files during any of the three build phases.
This section discusses variables that have special meaning to b2.
All of these must be defined or used in the global module -- using those
variables inside a named module will not have the desired effect. See
Modules.
These two variables control the binding of file target names to locations
in the file system. Generally, $(SEARCH) is used to
find existing sources while $(LOCATE) is used to fix
the location for built targets.
Rooted (absolute path) file targets are bound as is. Unrooted file target
names are also normally bound as is, and thus relative to the current
directory, but the settings of $(LOCATE) and $(SEARCH)
alter this:
$(LOCATE) is set then the target is bound relative
to the first directory in $(LOCATE). Only the
first element is used for binding.
$(SEARCH) is set then the target is bound to
the first directory in $(SEARCH) where the target
file already exists.
$(SEARCH) search fails, the target is bound
relative to the current directory anyhow.
Both $(SEARCH) and $(LOCATE) should
be set target-specific and not globally. If they were set globally,
b2 would use the same paths for all file binding,
which is not likely to produce sane results. When writing your own rules,
especially ones not built upon those in Jambase, you may need to set
$(SEARCH) or $(LOCATE) directly.
Almost all of the rules defined in Jambase set $(SEARCH)
and $(LOCATE) to sensible values for sources they
are looking for and targets they create, respectively.
These two variables control header file scanning. $(HDRSCAN)
is an egrep(1) pattern, with ()'s surrounding the
file name, used to find file inclusion statements in source files. Jambase
uses $(HDRPATTERN) as the pattern for $(HDRSCAN).
$(HDRRULE) is the name of a rule to invoke with the
results of the scan: the scanned file is the target, the found files
are the sources. This is the only place where b2 invokes
a rule through a variable setting.
Both $(HDRSCAN) and $(HDRRULE)
must be set for header file scanning to take place, and they should be
set target-specific and not globally. If they were set globally, all
files, including executables and libraries, would be scanned for header
file include statements.
The scanning for header file inclusions is not exact, but it is at least
dynamic, so there is no need to run something like makedepend(GNU)
to create a static dependency file. The scanning mechanism errs on the
side of inclusion (i.e., it is more likely to return filenames that are
not actually used by the compiler than to miss include files) because
it can't tell if #include
lines are inside #ifdefs
or other conditional logic. In Jambase, HdrRule
applies the NOCARE rule to each header file found
during scanning so that if the file isn't present yet doesn't cause the
compilation to fail, b2 won't care.
Also, scanning for regular expressions only works where the included
file name is literally in the source file. It can't handle languages
that allow including files using variable names (as the Jam
language itself does).
It is sometimes desirable to disallow parallel execution of some actions. For example:
Craig McPeeters has extended Perforce Jam to solve such problems, and that extension was integrated in Boost.Jam.
Any target can be assigned a semaphore, by setting
a variable called SEMAPHORE on that target. The value
of the variable is the semaphore name. It must be different from names
of any declared target, but is arbitrary otherwise.
The semantic of semaphores is that in a group of targets which have the
same semaphore, only one can be updated at the moment, regardless of
"-j" option.
A number of Jam built-in variables can be used to identify runtime platform:
OSOS identifier string
OSPLATUnderlying architecture, when applicable
MACtrue on MAC platform
NTtrue on NT platform
OS2true on OS2 platform
UNIXtrue on Unix platforms
VMStrue on VMS platform
JAMDATE
Time and date at b2 start-up as an ISO-8601
UTC value.
JAMUNAMEOuput of uname(1) command (Unix only)
JAMVERSION
b2 version, currently "3.1.19"
JAM_VERSION
A predefined global variable with two elements indicates the version
number of Boost Jam. Boost Jam versions start at "03"
"00". Earlier versions of Jam
do not automatically define JAM_VERSION.
When b2 executes a rule's action block, it forks and
execs a shell, passing the action block as an argument to the shell.
The invocation of the shell can be controlled by $(JAMSHELL).
The default on Unix is, for example:
JAMSHELL = /bin/sh -c % ;
The % is replaced with the text of the action block.
B2 does not directly support building in parallel
across multiple hosts, since that is heavily dependent on the local environment.
To build in parallel across multiple hosts, you need to write your own
shell that provides access to the multiple hosts. You then reset $(JAMSHELL)
to reference it.
Just as b2 expands a % to be the
text of the rule's action block, it expands a ! to
be the multi-process slot number. The slot number varies between 1 and
the number of concurrent jobs permitted by the -j
flag given on the command line. Armed with this, it is possible to write
a multiple host shell. For example:
#!/bin/sh # This sample JAMSHELL uses the SunOS on(1) command to execute a # command string with an identical environment on another host. # Set JAMSHELL = jamshell ! % # # where jamshell is the name of this shell file. # # This version handles up to -j6; after that they get executed # locally. case $1 in 1|4) on winken sh -c "$2";; 2|5) on blinken sh -c "$2";; 3|6) on nod sh -c "$2";; *) eval "$2";; esac
The __TIMING_RULE__ and __ACTION_RULE__
can be set to the name of a rule for b2 to call after an action completes for a target. They both
give diagnostic information about the action that completed. For __TIMING_RULE__
the rule is called as:
rule timing-rule ( args * : target : start end user system )
And __ACTION_RULE__ is called as:
rule action-rule ( args * : target : command status start end user system : output ? )
The arguments for both are:
args
Any values following the rule name in the __TIMING_RULE__
or __ACTION_RULE__ are passed along here.
target
The b2 target that was built.
commandThe text of the executed command in the action body.
statusThe integer result of the executed command.
startThe starting timestamp of the executed command as a ISO-8601 UTC value.
endThe completion timestamp of the executed command as a ISO-8601 UTC value.
userThe number of user CPU seconds the executed command spent as a floating point value.
systemThe number of system CPU seconds the executed command spent as a floating point value.
output
The output of the command as a single string. The content of the
output reflects the use of the -pX option.
If both variables are set for a target both are called, first __TIMING_RULE__
then __ACTION_RULE__.
Boost Jam introduces support for modules, which provide some rudimentary
namespace protection for rules and variables. A new keyword, "module"
was also introduced. The features described in this section are primitives,
meaning that they are meant to provide the operations needed to write Jam
rules which provide a more elegant module interface.
module expression { ... }
Code within the { ... } executes within the module named
by evaluating expression. Rule definitions can be found in the module's
own namespace, and in the namespace of the global module as module-name.rule-name,
so within a module, other rules in that module may always be invoked without
qualification:
module my_module { rule salute ( x ) { ECHO $(x), world ; } rule greet ( ) { salute hello ; } greet ; } my_module.salute goodbye ;
When an invoked rule is not found in the current module's namespace, it is looked up in the namespace of the global module, so qualified calls work across modules:
module your_module
{
rule bedtime ( ) { my_module.salute goodnight ; }
}
Each module has its own set of dynamically nested variable scopes. When execution passes from module A to module B, all the variable bindings from A become unavailable, and are replaced by the bindings that belong to B. This applies equally to local and global variables:
module A
{
x = 1 ;
rule f ( )
{
local y = 999 ; # becomes visible again when B.f calls A.g
B.f ;
}
rule g ( )
{
ECHO $(y) ; # prints "999"
}
}
module B
{
y = 2 ;
rule f ( )
{
ECHO $(y) ; # always prints "2"
A.g ;
}
}
The only way to access another module's variables is by entering that module:
rule peek ( module-name ? : variables + )
{
module $(module-name)
{
return $($(>)) ;
}
}
Note that because existing variable bindings change whenever a new module
scope is entered, argument bindings become unavailable. That explains the
use of "$(>)" in the peek rule above.
local rule rulename...
The rule is declared locally to the current module. It is not entered in the global module with qualification, and its name will not appear in the result of:
[ RULENAMES module-name ]
rule RULENAMES ( module ? )
Returns a list of the names of all non-local rules in the given module. If module is omitted, the names of all non-local rules in the global module are returned.
rule VARNAMES ( module ? )
Returns a list of the names of all variable bindings in the given module. If module is omitted, the names of all variable bindings in the global module are returned.
This includes any local variables in rules from the call stack which
have not returned at the time of the VARNAMES invocation.
IMPORT allows rule name aliasing across modules:
rule IMPORT ( source_module ? : source_rules * : target_module ? : target_rules * )
The IMPORT rule copies rules from the source_module
into the target_module as local rules. If either
source_module or target_module
is not supplied, it refers to the global module. source_rules
specifies which rules from the source_module to import;
target_rules specifies the names to give those rules
in target_module. If source_rules
contains a name which doesn't correspond to a rule in source_module,
or if it contains a different number of items than target_rules,
an error is issued. For example,
# import m1.rule1 into m2 as local rule m1-rule1. IMPORT m1 : rule1 : m2 : m1-rule1 ; # import all non-local rules from m1 into m2 IMPORT m1 : [ RULENAMES m1 ] : m2 : [ RULENAMES m1 ] ;
EXPORT allows rule name aliasing across modules:
rule EXPORT ( module ? : rules * )
The EXPORT rule marks rules from
the source_module as non-local (and thus exportable).
If an element of rules does not name a rule in module,
an error is issued. For example,
module X {
local rule r { ECHO X.r ; }
}
IMPORT X : r : : r ; # error - r is local in X
EXPORT X : r ;
IMPORT X : r : : r ; # OK.
rule CALLER_MODULE ( levels ? )
CALLER_MODULE returns the name of the module scope enclosing
the call to its caller (if levels is supplied, it is interpreted as an
integer number of additional levels of call stack to traverse to locate
the module). If the scope belongs to the global module, or if no such module
exists, returns the empty list. For example, the following prints "{Y}
{X}":
module X {
rule get-caller { return [ CALLER_MODULE ] ; }
rule get-caller's-caller { return [ CALLER_MODULE 1 ] ; }
rule call-Y { return Y.call-X2 ; }
}
module Y {
rule call-X { return X.get-caller ; }
rule call-X2 { return X.get-caller's-caller ; }
}
callers = [ X.get-caller ] [ Y.call-X ] [ X.call-Y ] ;
ECHO {$(callers)} ;
rule DELETE_MODULE ( module ? )
DELETE_MODULE removes all of the variable bindings and
otherwise-unreferenced rules from the given module (or the global module,
if no module is supplied), and returns their memory to the system.
Though it won't affect rules that are currently executing until they
complete, DELETE_MODULE should be used with extreme
care because it will wipe out any others and all variable (including
locals in that module) immediately. Because of the way dynamic binding
works, variables which are shadowed by locals will not be destroyed,
so the results can be really unpredictable.